Bob (physics)

A bob is the weight on the end of a pendulum. The use of a weight concentrated in a relatively small, compact object enables the centre of gravity to be positioned close to the physical end of the pendulum, which minimises the length of pendulum required for a given period. To function efficiently, a bob must have minimal resistance to air (if the pendulum operates in air) to reduce the energy losses in the system. Traditionally, a pendulum bob is round, and lens-shaped in section, but bobs in older pendulum clocks often have decorative carving and shapes characteristic of the type of clock. The name 'bob' may have originated just after the pendulum clock was invented in 1657. Early verge clock pendulums were short and had very wide swings of 80° or more. They were often mounted on the back of clock cases, and when running the weight appeared to 'bob' out from behind the case on each side. These became known as bob pendulums.^[1] Possibly the term spread to all pendulum weights. In older clocks, the bob is attached to the pendulum using a threaded rod with an adjustment nut on its distal end. Moving the nut upwards causes the pendulum to beat more quickly, making the clock run faster. The weight of the bob does not itself affect the period of the pendulum. However, a heavier bob (within reason) helps to carry forward the action of the clock escapement until it receives an additional push as the escapement re-engages. Too light a bob may be insufficient to overcome and maintain the motion of the pendulum for a long period of time.

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